Reclosable thermoplastic bags with zipper-type closures have come into widespread use. The problem addressed by the present invention pertains to the determination of whether or not the zipper of a reclosable plastic bag is properly closed or "zipped". Such a determination is often difficult save with a close visual inspection of the zipper.
One known way of making the determination easier is to use a zipper which provides a color change when the zipper is properly zipped. Different colors are utilized in each of the opposing zipper profiles to produce a third distinct color when the profiles are interlocked. Zippers utilizing such a color change are seen in U.S. Pat. Nos. 4,186,786 and 4,285,105. The disadvantage of using such a color change is that it may occur when the opposing zipper profiles are closely overlapped instead of only when the profiles are properly interlocked.